Canada - Industrial - Ampacity for Single Core Parallel Run of Low Voltage Cables

[j_p_c]

Hi Eng-Tips Electrical Community,

This is my first post though I'll admit I've been a ghost-reader for some time. Many thanks for all the helpful posters I've benefited from in the past.

I'm an electrical engineer working on a low voltage transformer secondary cable application and am seeking advice on cable ampacity as per the CEC.

Details:

- 600V, 3 wire.
- Single core TECK90, sheaths bonded at source only.
- Run Length: 10m
- Non-ferrous gland plates to be used at entries.
- Blocking spacing for short circuit forces (to be determined).
- Required ampacity: 2000A
- Proposed configuration: 4 per phase 1c500MCM in trefoil arrangements (each ABC) with 1 cable diameter spacing between cables, and 2 cable diameter spacing between trefoil groups.


Question:

I'm unclear on the application of Table 1 ampacities as they relate to parallel runs of single conductor cables. The appendix for rule 12-108 indicates that a trefoil configuration (cores touching) for each per phase conductor will allow for even splitting of current between the parallel cores (see figure B12-1). However, the appendix notes that the configurations shown may not always result in equal current division, with little explanation. Moreover, I'd prefer to space the cables in each trefoil by 1 cable diameter to take advantage of full single core ampacity if possible. However, I'm not clear if this has an impact on the mutual inductance between each trefoil. Some other references indicate that no more than one cable diameter spacing in trefoil, should still result in relatively even current division between parallel sets, but the CEC is silent on this. (or is it?). I could also do a two layer configuration that is also purported to lead to equal current division (other references), but that would require a derating for layering, which I can infer from table 5d, but for which I don't believe there is an obvious rule.

Parallel sets of single core cables are done so often in industrial applications that I'm thinking I may be missing something. I'd expected the CEC to provide a straightforward method for installation method and derating requirements for single core parallel installations. Am I missing something obvious? I just completed a Neher McGrath calculation for another project, and I'm trying to avoid the complexity and cost by hopefully applying the CEC alone.

Sincere Thanks,
Justin

 

[waross]

Will column 2 of Table 1 and Table 5B help?
I wouldn't worry about current division for a 10 M run.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[j_p_c]

Thanks Bill,

Table 1, col 2 are what I'm after! As I'm >=100% spacing I shouldn't have to derate by table 5b no? I'm still a bit worried about current sharing, but can see why you don't think it's a big deal for such a short run. I guess my concern was that for such a short run the impedance will be very low for each parallel path and as such small differences between the two could lead to unexpectedly large deviations in current splitting.

Cheers,
Justin

 

[waross]

The current per cable will be 500 Amps.
Table 1, Column 2 allows 620 Amps.
The worst case will be a split of 620 Amps and 380 Amps.
I can't see cable arrangements causing anywhere near that amount of unbalance.
Anecdote/suggestion
Rather than the expense of non-ferrous plates, I have made hacksaw cuts between each entry hole in each group.
I used non-ferrous connectors and ordered aluminum locknuts.
I didn't get the aluminum locknuts but the inspector didn't notice and the locknuts didn't seem to overheat.
The company had a similar installation and the supervisor/purchasing agent checked with me before issuing orders to the second crew.
I emphasized again that the locknuts should be aluminum.
He still didn't get the message.
The inspector called him on the locknuts.
The concentric grounds were twisted together and terminated at the supply end.
Changing the locknuts was less than straightforward.
He came crying to me about the wasted labour changing the locknuts.
I reminded him that I had told him at least six times that he needed aluminum locknuts.
Even with non-ferrous plates you may be called if you use ferrous connectors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[j_p_c]

Thanks Bill for the detailed pointers, for some reason I'm a chicken to provide the hacksaw cuts (except for brown-field quick mod), though totally would work once you introduce the airgap flux impedance - MacGyver would not approve of my lame impracticality

 

[waross]

Glad to help.
I have done this mostly on new installs,
Once I was able to see a dramatic reduction in heating when hacksaw cuts were added to an existing problem installation.
Ps: Use a power saw such as a Saws-All or a metal cutting Skill-Saw.

Bill
--------------------
"Why not the best?"
Jimmy Carter